[PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Robin Murphy]

For dma_map_sg(), DMA API implementations are free to merge consecutive
segments into a single DMA mapping if conditions are suitable, thus the
resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
its callers either have to reject the 0 < count < nents case or risk
getting bogus addresses back later. Fortunately this is relatively easy
to deal with having to rejig structures to also store the mapped count,
since the total DMA length should still be equal to the total buffer
length. All we need is a separate scatterlist cursor to iterate the DMA
addresses separately from the CPU addresses.

Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---

Off the back of Sinan's proposal for a workaround, I took a closer look
and this jumped out - I have no hardware to test it, nor do I really
know my way around this code, so I'm probably missing something, but at
face value this seems like the only obvious problem, and worth fixing
either way.

These patches are based on drm-next, and compile-tested (for arm64) only.

Robin.

 drivers/gpu/drm/drm_prime.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 7856a9b3f8a8..db3dc8489afc 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -933,16 +933,18 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
 				     dma_addr_t *addrs, int max_entries)
 {
 	unsigned count;
-	struct scatterlist *sg;
+	struct scatterlist *sg, *dma_sg;
 	struct page *page;
-	u32 len, index;
+	u32 len, dma_len, index;
 	dma_addr_t addr;
 
 	index = 0;
+	dma_sg = sgt->sgl;
+	dma_len = sg_dma_len(dma_sg);
+	addr = sg_dma_address(dma_sg);
 	for_each_sg(sgt->sgl, sg, sgt->nents, count) {
 		len = sg->length;
 		page = sg_page(sg);
-		addr = sg_dma_address(sg);
 
 		while (len > 0) {
 			if (WARN_ON(index >= max_entries))
@@ -957,6 +959,12 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
 			len -= PAGE_SIZE;
 			index++;
 		}
+
+		if (dma_len == 0) {
+			dma_sg = sg_next(dma_sg);
+			dma_len = sg_dma_len(dma_sg);
+			addr = sg_dma_address(dma_sg);
+		}
 	}
 	return 0;
 }
-- 
2.16.1.dirty